1 Digital and Film Radiography Comparison and Contrast Reference Handout A Special Seminar for the ASNT Fall 2015 Conference Terms and Definitions Direct Radiography (DR)* Radiography that converts radiation directly to stored image data. Indirect Radiography Radiography that uses an intermediate storage medium prior to storage of the image data. *Overloaded Acronyms and terms Acronyms and terms are often used to mean different things, and we need to be careful to try to communicate clearly. For example: 2 Cameras 3 Terms & Definitions Film Radiography (RT) A form of radiographic imaging, where photographic film is exposed to radiation transmitted through an item being inspected, and light or radioactive rays, an invisible image (called a latent image) and a latent image is formed in the emulsion layer of the film. Conversion of the latent image to a visible image is through chemical processing.
2 Traditional Film Radiography must use more radiation to produce an image of similar Contrast to Digital methods. The image is stored on a sheet of radiographic film which is viewed based on the transmission of light through the film. 4 Terms and Definitions Digital Radiography (DR) A form of radiographic imaging in which Digital detectors are exposed to the radiation transmitted through an item being inspected, and convert the transmission data to a Digital file to be stored and displayed on a computer. A form of radiographic imaging, where Digital radiographic sensors are used instead of traditional radiographic film. Advantages include time efficiency through bypassing chemical processing and the ability to digitally transfer and enhance images. Also, in most cases, less radiation can be used to produce an image of similar Contrast to film Radiography .
3 Instead of radiographic film, Digital Radiography uses a Digital image capture device. This gives advantages of immediate image preview and availability; elimination of costly film processing steps; a wider dynamic range, which makes it more forgiving for over- and under-exposure; as well as the ability to apply special image processing techniques that enhance overall display of the image. Terms and Definitions Computed Radiography (CR) Digital Radiography using storage phosphor plates as an intermediate storage media prior to scanning the plate to create a Digital image. Implementation is similar to film Radiography except that in place of a film to create the image, an imaging plate (IP) made of photostimulable phosphor is used. The imaging plate is housed in a cassette and placed under the body part or object to be examined and the radiographic exposure is made.
4 Hence, instead of taking an exposed film into a darkroom for developing in chemical tanks or an automatic film processor, the imaging plate is run through a special laser scanner, or CR reader, that reads and digitizes the image. 6 Terms and Definitions Computed Tomography (CT) Digital Radiography that allows data to be displayed in a non standard manner. Implementation requires additional image acquisition, additional handling hardware, and complex image processing algorithms Makes use of computer-processed combinations of many radiographic images taken from different angles to produce cross-sectional (tomographic) images (virtual 'slices') of specific areas of a scanned object, allowing the user to see inside the object without cutting. Image processing is used to generate a three-dimensional image of the inside of the object from a large series of two-dimensional radiographic images taken around a single axis of rotation.
5 7 Film vs Digital Comparison 8 Film Digital Development time Faster (immediate or near immediate results) Technology and equipment well known and understood Technology continuously developing and not as well understood Requires HAZMAT chemical processing No HAZMAT chemicals Physical transfer of film Electronic transfer of data Simpler and better known process controls More complex for setup and process controls Wider dynamic range Ability to process and manipulate data Requires less radiation and time for exposures Radiation Spectrum Radiographic inspection utilizes energy levels exceeding ultraviolet light Develops penetrating, and ionizing capabilities Causes a reaction in the detector used to develop an observable image Areas of higher density will allow less radiation to pass through, while areas of lower density will allow more radiation to pass through.
6 This creates an image of defects of higher or lower density than the material around it. 9 Nyquist-Shannon Sampling Theorem This theorem is typically utilized in Digital Radiography to answer the all important question: What resolution do I need to see the level of detail required for a given flaw size? 10 Definition (abbreviated): In the field of Digital signal processing, the sampling theorem is a fundamental bridge between continuous-time signals (often called "analog signals") and discrete-time signals (often called " Digital signals"). It establishes a sufficient condition for a sample rate that permits a discrete sequence of samples to capture all the information from a continuous-time signal of finite bandwidth. 1 The sampling theorem introduces the concept of a sample rate that is sufficient for perfect fidelity for the class of functions that are bandlimited to a given bandwidth, such that no actual information is lost in the sampling process.
7 It expresses the sufficient sample rate in terms of the bandwidth for the class of functions. The theorem also leads to a formula for perfectly reconstructing the original continuous-time function from the samples. 1 1 Excerpts from Wikipedia, the free encyclopedia. 11 Nyquist-Shannon Sampling Theorem Nyquist-Shannon Sampling Theorem Critical frequency . 1 To illustrate the necessity of fs > 2B, consider the family of sinusoids (depicted in Fig. 8) generated by different values of in this formula: With fs = 2B or equivalently T = 1/(2B), the samples are given by: 12 Nyquist-Shannon sampling theorem Pixel pitch = Nyquist frequency = LP/mm Examples 50 m pixel pitch: NF = 10 LP/mm 100 m pixel pitch: NF = 5 LP/mm 200 m pixel pitch: NF = LP/mm 13 What is Noise?
8 In Film Radiography noise is caused by scatter, is often referred to as graininess and can be corrected by using a finer film. In DR noise is caused by scatter, undesired radiation and other electronic effects that vary by the type of detector. In Digital techniques, a number on a brightness scale is assigned to a particular value of radiation dose to create an image, when unwanted radiation is present, it takes up those values, reducing the range of values and sensitivity of the test. 14 Viewing Noise on a Histogram A histogram is a representation of all values assigned to every level of dose received on the Digital detector. If a particular system was capable of assigning 65,536 gray scale values, a test that assigned 45,000 of those values to radiation that passed through the part would be more sensitive then a test that had more scatter and only assigned 25,000 values to the radiation that passed through the part.
9 15 Viewing Noise on a Histogram This can be seen by looking at the histogram and viewing the assigned gray scale values. 16 Noise Comparison 17 Source-Detection Optimization Film Digital Use proper energy to avoid over/under exposure Use proper energy and flux to avoid detector saturation (blooming effect) or damage Use of the cleanest source possible and shield detector system to reduce background and scatter problems Measurements at different energies can be used to increase/change Contrast (ratio of radiographs at different energies for example) Depth-of-field can be adjusted with distance scintillator/converter-to-sensor 18 19 DR SYSTEM COMPONENTS Computed Radiography How is CR Different Than Film The CR Image Plate (IP) functions in a completely different fashion than film CR has substantially wider latitude than film The exposure process requires a different approach partly due to scatter The CR scanner functions in a completely different fashion than film processor The CR image is displayed on a computer monitor instead of a light box Software tools are used to adjust the image in ways film can t be changed Image quality measured by Signal to Noise Ratio (SNR) using software tools Additional operator training is required THE ENTIRE PROCESS OF CREATING AN IMAGE USING Computed Radiography IS DIFFERENT THAN FILM !
10 CCD CCD: Charge Coupled Device Advantages: Large field of view (FOV) High Quantum Efficiency (QE) Different speed read-out (up to a few MHz) Disadvantages: Relatively slow system (seconds acquisition time) Lengthy transfer of the data causing dead time between acquisition of radiograph (seconds) Dark current not negligible (reduced when CCD is cooled) 21 Detector Comparison Film Digital Film Speed CR and DR generally faster requiring lower dose. Limited Latitude. Logarithmic film curves. CR and DDA s have extremely high latitude (bit depth) and linear response. 12, 16, or 32-bit gray scale Automatic or manual film processing CR Scanning of IPs. Direct link on DDA s Computer Image Processing to produce images. Reliance on Chemicals and Clean processing Manual Film Cassette Handling Automation allows CT scans in seconds to minutes, Radiography in fractions of seconds CR can be setup to handle same as film.